1. Field of the Invention
The present invention relates to a plug cap device for an internal combustion engine for connection to a spark plug used in the internal combustion engine.
2. Description of the Prior Art
A plug cap device for carrying high voltage to a spark plug in an internal combustion engine has been of various constructions as disclosed in Japanese Utility Model Application Laid-Open No. 64-8580 (1989) and Japanese Patent Application Laid-Open No. 3-264776 (1991).
As shown in FIGS. 5 and 6, for example, a plug cap device 1 comprises a tubular plug cap body 2, an ignition cable 3 introduced into the plug cap body 2 at its one end, and an electric conductor 4 housed and held in the plug cap body 2 and connected to the ignition cable 3.
The plug cap body 2 is a flexible insulator formed of an elastic material such as silicone rubber.
The plug cap body 2 is press fitted at its second end over a spark plug 6 to connect a terminal portion of the spark plug 6 to the electric conductor 4.
In avoidance of leaks between the inner peripheral surface of the plug cap body 2 and the outer peripheral surface of an insulator 7 of the spark plug 6, the inner diameter of the plug cap body 2 receiving the insulator 7 of the spark plug 6 is smaller than the outer diameter of the insulator 7 so that the outer peripheral surface of the insulator 7 closely contacts the inner peripheral surface of the plug cap body 2 for exhibition of an electrical sealing property.
For example, when the outer diameter .phi.D1 of a major diameter portion of a corrugation portion 7a of the insulator 7 is 10.2 mm and the outer diameter .phi.D2 of a cylindrical portion 7b of the insulator 7 is 10.5 mm, the inner peripheral surface diameter .phi.A of the plug cap body 2 receiving the spark plug 6 is 9.5 mm which is smaller than the outer diameter .phi.D1 of the major diameter potion of the corrugation portion 7a. The inner diameter of the plug cap body 2 is of the same size or slightly increases gradually in a tapered configuration in a portion corresponding to the insertion position of the cylindrical portion 7b toward an insertion cud at which the spark plug 6 is relatively inserted into the plug cap body 2. The inner diameter .phi.B of the plug cap body 2 at the insertion end is 9.9 min.
Recently, a need has arisen to improve the electrical sealing property with increasing voltage required for the spark plug 6.
For improvement in the electrical sealing property, the plug cap body 2 has been designed to have a smaller inner diameter .phi.A to squeeze the insulator 7 of the spark plug 6 more tightly.
For example, when the outer diameter .phi.D1 of the major diameter portion of the corrugation portion 7a of the insulator 7 is 10.2 mm and the outer diameter .phi.D2 of the cylindrical portion 7b of the insulator 7 is 10.5 mm, the inner peripheral surface diameter .phi.A of the plug cap body 2 receiving the spark plug 6 is 8.6 mm which is smaller than the outer diameter .phi.D1 of the major diameter portion of the corrugation portion 7a. The inner diameter of the plug cap body 2 slightly increases gradually toward the insertion end in a tapered configuration in the portion corresponding to the insertion position of the cylindrical portion 7b. The inner diameter .phi.B of the plug cap body 2 at the insertion end is 9.0 mm.
The corrugation portion 7a and cylindrical portion 7b of the insulator 7 are relatively inserted into the plug cap body 2 having the inner diameters .phi.A and .phi.B which are smaller than the outer diameters .phi.D1 and .phi.D2 of the insulator 7 of the spark plug 6. This provides the improved electrical sealing property but requires a greater force to insert the insulator 7 of the spark plug 6 into the plug cap body 2. In particular, when removed from the spark plug 6, the plug cap body 2 acts as a sucking-disc and an abnormally great force is required to remove the plug cap body 2 from the spark plug 6, resulting in very low operating efficiency.